1. Field of the Invention
The present invention relates to a method of rate matching process to improve the hybrid Automatic Repeat reQuestion (HARQ) operation, and more particularly, to a method of rate matching process of the hybrid Automatic Repeat reQuestion (HARQ) operation in which coded bits are selected for a transmission depending on whether the transmission carries the first transmission of a new packet or a retransmission for an existing packet.
2. Description of the Related Art
Currently, hybrid Automatic Repeat reQuestion (HARQ) is widely used in communication systems to combat decoding failure and improve reliability of data communication. In a data communication system, each data packet is protected by using a certain forward error correction (FEC) coding scheme. Each subpacket may only contain a portion of coded bits of the entire packet. The process of selecting coded bits to be transmitted in a subpacket is called subpacket generation, or rate matching.
In a contemporary HARQ operation, a data packet is coded by using an encoder with a certain kind of Forward Error Correction (FEC) scheme. The data packet is processed by a subpacket generation/rate matching stage and then a set of subpackets are generated. A subpacket, for example, a subpacket k may only contain a portion of the coded bits. If the transmission by a transceiver for subpacket k fails, as indicated by a negative acknowledgement (NAK) message provided by a feedback acknowledgement channel, a retransmission subpacket, subpacket k+1, is provided to retry the transceive of this data packet. If subpacket k+1 is successfully transceived, an acknowledgement (ACK) message is provided by feedback acknowledgement channel. The retransmission subpackets may contain different coded bits from one subpacket. The receiver may softly combine or jointly decode all the received subpackets by a decoder to improve the chance of decoding. Normally, a maximum number of transmissions of a packet is configured in consideration of both reliability, packet delay, and implementation complexity.
N-channel synchronous hybrid Automatic Repeat reQuestion (HARQ) is widely used in wireless communication systems because of the simplicity of the N-channel synchronous hybrid Automatic Repeat reQuestion (HARQ). For example, synchronous hybrid Automatic Repeat reQuestion (HARQ) has been accepted as the hybrid Automatic Repeat reQuestion (HARQ) scheme for long term evolution (LTE) uplink in 3rd Generation Partnership Project (3GPP).
Because of a fixed timing relationship between subsequent transmissions, the transmission slots in an individual hybrid Automatic Repeat reQuestion (HARQ) channel exhibits an interlace structure. When a packet is correctly decoded, the receiver sends back an acknowledgement (ACK) to the transmitter. The transmitter then starts transmitting a new packet at a next slot in the current interlace. Otherwise, the transmitter receives a negative acknowledgement (NAK) from the receiver, and the transmitter transmits another sub-packet of the same packet at the next slot in the current interlace. Different from synchronous hybrid Automatic Repeat reQuestion (HARQ), asynchronous hybrid Automatic Repeat reQuestion (HARQ) may be used to provide more flexibility in scheduling and multiplexing different users. With asynchronous hybrid Automatic Repeat reQuestion (HARQ), the time interval between transmissions of the same packet may not be fixed. The transmitter has the freedom to decide when a packet is retransmitted.
If the resource assignment, or modulation schemes, may be changed during retransmissions, the hybrid Automatic Repeat reQuestion (HARQ) operation is adaptive. In long term evolution (LTE) systems, the hybrid Automatic Repeat reQuestion (HARQ) operation may be both asynchronous and adaptive. Sometimes a receiver might have difficulty in detecting the packet boundary, i.e., difficulty in detecting whether a subpacket is a first sub-packet of a new packet to be transmitted or a retransmission sub-packet of a previous packet. To alleviate this problem, a new packet indicator may be transmitted in the control channel that carries transmission format information for the packet. Sometimes, a more elaborated version of hybrid Automatic Repeat reQuestion (HARQ) channel information, such as sub-packet identifier (ID), or even hybrid Automatic Repeat reQuestion (HARQ) channel ID, may be transmitted to help the receiver detect and decode the packet.
In a long term evolution (LTE) system, when the size of transport block is large, the transport block is segmented into multiple code blocks so that multiple coded packets may be generated, which is advantageous because of benefits such as enabling pipeline or parallel processing implementation and flexible trade off between power consumption and hardware complexity. In the case of multiple code blocks, a rate matching process may be executed for per code block. During the rate matching process, the hybrid Automatic Repeat reQuestion (HARQ) functionality selects a number of bits at the output of the channel coder to match the total number of bits that the physical channel can carry. The hybrid Automatic Repeat reQuestion (HARQ) functionality is controlled by redundancy version (RV) parameters. The exact set of bits at the output of the hybrid Automatic Repeat reQuestion (HARQ) functionality depends on the number of input bits, the number of output bits, and the redundancy version (RV) parameters. A circular buffer based rate matching process is adopted in the long term evolution (LTE) systems.
Each code block C may be separated, at the turbo encoder output, into a systematic bit stream S, a first parity stream P1, and a second parity stream P2. Four redundancy versions (RVs) may be defined, each of which specifies a starting bit index in the buffer. The transmitter chooses one RV for each hybrid Automatic Repeat reQuestion (HARQ) transmission. The transmitter reads a block of coded bits from the buffer, starting from the bit index specified by a chosen RV while removing filler bits and dummy bits. If the maximum capacity of the buffer is reached and more coded bits are needed for transmission, the transmitter wraps around and continues at the beginning of the buffer, hence the term of “circular buffer” is generated.
The circular buffer based rate matching may be implemented in long term evolution (LTE) for downlink shared channel (DL_SCH) and uplink shared channel (UL_SCH).
A receiver however sometimes may have difficulties in detecting the packet boundary, i.e., difficulty in defining whether a subpacket is the first sub-packet of a new packet or a retransmission sub-packet. Failures of informing the receiver a transmission of a new packet may either degrade the efficiency of packet transmitting and coding rate through the re-transmission, or induce deterioration of the quality of received packet.